It depends on the type of Mac you're upgrading. I have a late 2009 27" iMac that I "upgraded" prior to leaving for work this morning. The maddening this is that while I have no issues with my Bluetooth keyboard, I can only move the mouse cursor with my Magic Trackpad. When I checked System Preferences, the Bluetooth icon was simple not there! I saw the Bluetooth icon in the Menu Bar, but the message stated that Bluetooth was unavailable!
I don't know what's going on here, but I'm going to go out on a limb and say that integrated systems (ie., laptops only) are probably pretty safe with this "upgrade", but iMacs and Mac Pros could be a bit dicy. I'm not sure what to do at this point, but will probably attempt a re-install, and should that fail, go ahead and do a "nuke and pave." After all, it's only been six years since I made the switch - I still remember that option from my Windows days
Same for me. Went very slowly (3 mbs DSL), but no problems anywhere along the way. Everything seems to be working quite well. Am currently downloading updates to the iWorks, etc., series for compatibility.